1. Field of the Invention
The present invention relates to an electrophotographic apparatus for forming an electrostatic image by charging a photosensitive member, exposing the photosensitive member to light to form an electrostatic image, and developing such electrostatic image with developer.
2. Related Background Art
Recent development and the spread of computers have increased the demand for printers as output means for the computers, and commercialized digital copying apparatus that combine the functions of a printer with those of a conventional copying machine have been developed.
In such digital copying apparatus, the photosensitive material constituting the photosensitive layer of the electrophotographic photosensitive member (for example, a photosensitive drum) is composed of an inorganic photoconductive material such as selenium or amorphous silicon (a-Si), or an organic photoconductive material.
Amorphous silicon (a-Si), which is an inorganic photoconductive material, is used in the photosensitive member in the form of a non-monocrystalline deposition film principally composed of silicon and further containing hydrogen and a halogen such as fluorine or chlorine, and is proposed as a photosensitive member of high performance, high durability and no pollution, and is used in high-speed or large-scale apparatus for volume printing.
Non-monocrystalline silicon-based deposition film can be formed by many methods, such as sputtering by bombarding a target with an ion beam, thermal CVD by decomposing raw material gas with heat, photo CVD by decomposing raw material gas with light, and plasma CVD by decomposing raw material gas with plasma.
Among these methods, the plasma CVD (PCVD) method forms a deposition film on the surface of a desired substrate such as glass, quartz, heat-resistant plastic film, stainless steel or aluminum by generating glow discharge by a DC current or a high frequency current (RF or VHF) or microwave to form plasma and decomposing raw material gas with such plasma. This method is practiced not only for the formation of a-Si deposition film for the electrophotographic photosensitive member but also for the formation of deposition films for other purposes, and various apparatus for this method are being proposed.
In a typical PCVD apparatus, the substrate is vertically set in a vertical reaction chamber, and the raw material gas is introduced to the vicinity of the substrate and is decomposed and deposited by plasma, and the used gas is discharged from a discharge aperture of the reaction chamber. The apparatus is principally classified into three types according to the position of discharge of the used gas.
Among these, a type which discharges used gas from the discharge aperture at the lateral wall of the reaction chamber and a type which discharges used gas from the discharge aperture provided in the upper part of the lateral wall of the reaction chamber are associated with a danger of intrusion of contaminants from the exterior into the reaction chamber through the discharge aperture and also with a drawback in that a highly pure a-Si deposition film, free from contaminant, cannot be obtained. On the other hand, a type which discharges used gas from the discharge aperture at the bottom of the reaction chamber is not associated with such drawback, but the obtained a-Si deposition film often has a slope in the vertical direction (longitudinal direction of photosensitive member) in certain characteristics such as the charging ability by the charger.
If the a-Si deposition film, or the a-Si photosensitive layer, has sloped characteristics, for example in the charging ability, in the longitudinal direction of the photosensitive member (namely in the vertical direction at the formation of the a-Si deposition film), the latent image formed by charging and image exposure becomes uneven. More specifically, a portion, having a higher charging ability, in the photosensitive layer shows a higher sensitivity to the image exposure, so that the latent image formed in such portion shows a higher potential contrast and is excessively developed. In a digital copying apparatus, the exposure amount per dot can be made uniform over the longitudinal direction of the photosensitive member, so that the influence of such sloped characteristics in the photosensitive layer is noticed conspicuously.
On the other hand, in order to meet recent requirements for finer image quality, a digital image is formed with a dot size of 60 to 70 .mu.m by a digital process of 400 dpi or with a dot size of 40 to 50 .mu.m by a digital process of 600 dpi, and, for this purpose, there are conducted improvements in the characteristics of the photosensitive member and reduction in the particle size of the developer, both in one-component developer and two-component developer. A developer of small particle size clearly shows the difference between the 400 dpi process and the 600 dpi process mentioned above and is very effective in improving the image quality. More specifically, as a toner of small particle size to be used in the developer, it is preferred that the toner be of a weight-averaged particle size of 6 to 9 .mu.m, as measured for example by a Coulter counter, in order to precisely reproduce a dot of the latent image in the development operation while preventing fog.
There are known various developers according to these applications, but a one-component developer, consisting solely of toner, is being recently employed in a larger number of apparatus because it is not associated with the drawbacks of deterioration or replacement of the carrier and it allows the use of more compact developing unit.
In general, toner constituting a one-component developer is composed of resin particles of spherical or undefined shape with a weight-averaged particle size of 3 to 15 .mu.m, containing magnetic substance and prepared by a synthetic method or a crushing method. For obtaining desired functions, there is externally added a powdered substance of sub micron order in size. The magnetic substance is composed for example of magnetite, while the resinous substance is composed for example of polystyrene, polyester or polybutadiene, and the externally added substance is composed for example of alumina, silica or strontium titanate.
For achieving higher image quality, Japanese Patent Application Laid-Open No. 9-80855 discloses a method of appropriately controlling the developing bias to eliminate the influence of unevenness in the characteristics of the photosensitive member, thereby improving the image quality, but still higher image quality is being sought.
In a digital copying apparatus, since the digital image is formed by a group of dots, such as those of about 40 .mu.m by the 600 dpi digital process as explained in the foregoing, a deterioration in the reproducibility of the dot diameter in the developed image results in an unevenness in the image density, particularly in a halftone image.
In a low-speed printer and the like, it is possible to apply multi-level modulation to each dot thereby obtaining uniform image density or achieving improvement in the image gradation, but such a method is difficult to adopt in a high-speed printer because the digital process required therefor is cumbersome and difficult to adopt.
Also, the output image from the computer often contains fine hatchings, etc., and requires both a sufficiently high image density and a high sharpness. This requirement can be met for the non-contact developing method by reducing the gap between the developing sleeve of the developing unit and the photosensitive member.
However, if the photosensitive layer of a photosensitive member shows a sloped distribution in the characteristics thereof, unevenness in the potential of the photosensitive layer results in an unevenness in the dot diameter after development, thereby causing unevenness in the density of the developed image.